refactor: use errors.Is()/errors.AsType() for error comparison and rename shadowed variables

Replace direct error equality checks (err == pgx.ErrNoRows, err != http.ErrServerClosed)
with the idiomatic errors.Is() function throughout handlers, services, middleware, and app
startup. This correctly handles wrapped error chains.

Also replace a raw type assertion (*HTTPError) with errors.AsType[*HTTPError]() in the
error handler middleware for consistency.

Additionally, rename shadowed variables for clarity:
- sidecar.go: config -> sidecarConfig, systemConfig (shadowed package-level vars)
- media_scanner.go: uuid -> uuidString (shadowed the uuid package import)

internal/services/media_scanner.go

@@ -15,6 +15,7 @@ import (
 	"encoding/hex"
 	"encoding/json"
 	"encoding/xml"
+	"errors"
 	"fmt"
 	"image"
 	_ "image/jpeg"
@@ -590,7 +591,7 @@ func (s *MediaScanner) processMediaFile(ctx context.Context, path string) (bool,
 			fmt.Printf("Media item already exists with same size, skipping: %s\n", path)
 			return false, nil
 		}
-	} else if err != pgx.ErrNoRows {
+	} else if !errors.Is(err, pgx.ErrNoRows) {
 		fmt.Printf("Database error checking media item existence: %v\n", err)
 		return false, fmt.Errorf("failed to check if media item exists: %v", err)
 	}
@@ -2605,7 +2606,7 @@ func (s *MediaScanner) scanDirectory(ctx context.Context, dirPath string) {
 			LibraryID: libraryID,
 		})
 
-		if err == pgx.ErrNoRows {
+		if errors.Is(err, pgx.ErrNoRows) {
 			if _, err := s.processMediaFile(ctx, path); err != nil {
 				s.errors++
 			} else {
@@ -2780,7 +2781,7 @@ func (s *MediaScanner) SyncFilesystemWithDatabase(ctx context.Context) error {
 				FilePath:  relPath,
 				LibraryID: libraryID,
 			})
-			if err == pgx.ErrNoRows {
+			if errors.Is(err, pgx.ErrNoRows) {
 				// New file found - scan it
 				absPath := folder + "/" + relPath
 				if _, err := os.Stat(absPath); err == nil {
@@ -2848,19 +2849,19 @@ func (s *MediaScanner) extractOPFIdentifiers(epubPath string) (opfIdentifier, op
 		return "", "", "low", nil
 	}
 
-	var identifier, uuid string
+	var identifier, uuidString string
 	for _, id := range identifiers {
 		id = strings.TrimSpace(id)
 
 		// Check for UUID format (urn:uuid:)
 		if trimmed, found := strings.CutPrefix(strings.ToLower(id), "urn:uuid:"); found {
-			uuid = trimmed
+			uuidString = trimmed
 			continue
 		}
 
 		// Check if it's a plain UUID (8-4-4-4-12 format)
 		if isValidUUID(id) {
-			uuid = id
+			uuidString = id
 			continue
 		}
 
@@ -2879,9 +2880,9 @@ func (s *MediaScanner) extractOPFIdentifiers(epubPath string) (opfIdentifier, op
 		}
 	}
 
-	confidence = s.determineHashConfidence(uuid, identifier)
+	confidence = s.determineHashConfidence(uuidString, identifier)
 
-	return identifier, uuid, confidence, nil
+	return identifier, uuidString, confidence, nil
 }
 
 // isValidUUID checks if string is a valid UUID (8-4-4-4-12 format)

internal/services/search.go

@@ -3,6 +3,7 @@ package services
 import (
 	"bookhoard/internal/database"
 	"context"
+	"errors"
 	"strings"
 
 	"github.com/jackc/pgx/v5"
@@ -214,7 +215,7 @@ func (s *SearchService) SearchFieldValues(ctx context.Context, params FieldSearc
 // Shared between JSON endpoint and HTML rendering
 func (s *SearchService) ExecuteSearch(ctx context.Context, params SearchParams) ([]database.SearchMediaItemsUnifiedRow, int, error) {
 	results, err := s.SearchMediaItemsUnified(ctx, params)
-	if err != nil && err != pgx.ErrNoRows {
+	if err != nil && !errors.Is(err, pgx.ErrNoRows) {
 		return nil, 0, err
 	}